Distributed gain polygon ring laser amplification

ABSTRACT

A distributed gain polygon ring laser system includes a substrate ring, top and bottom cover plates, an input pump laser, an output coupler and a number of reflection points. The substrate ring has inner and outer surfaces. The top and bottom cover plates are configured for vacuum sealing with the substrate ring. The input pump laser is configured to direct light into the substrate ring. The plurality of reflection points are spaced around the inner surface of the substrate ring and are configured to reflect light from the input pump laser to the output coupler in a series of reflections.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of U.S. Provisional Application No. U.S. 63/294,490 filed on 29 Dec. 2021, pending, which is incorporated herein by reference in its entirety.

SUMMARY

The disclosure describes a distributed gain polygon ring laser amplifier. The laser amplifier includes a substrate ring having inner and outer surfaces and a plurality of reflection points spaced around the inner surface of the substrate ring and configured to reflect light from an input pump laser to an output coupler.

The disclosure also describes a master oscillator power amplifier laser system. The master oscillator power amplifier laser system includes a substrate ring having inner and outer surfaces, top and bottom cover plates configured for vacuum sealing with the substrate ring, an input pump laser configured to direct/fire/pump light into the substrate ring, an output coupler and a plurality of reflection points spaced around the inner surface of the substrate ring configured to reflect light from the input pump laser to the output coupler.

Further, the disclosure describes a ring laser reflection point. The ring laser reflection point includes laser media sandwiched between a laser mirror and an anti-reflective coating.

BRIEF DESCRIPTION OF THE FIGURES

The summary above, as well as the following detailed description of illustrative embodiments, is better understood when read in conjunction with the appended drawings. For the purpose of illustrating the disclosure, example constructions are shown in the drawings. However, the disclosure is not limited to specific methods and instrumentalities disclosed herein. Moreover, those having ordinary skill in the art will understand that the drawings are not to scale. Wherever possible, like elements have been indicated by identical numbers.

Embodiments of the disclosure will now be described, by way of example only, with reference to the following diagrams wherein:

FIG. 1 illustrates a top view of an example polygon ring laser amplifier suitable for use in association with disclosed master oscillator power amplifier laser systems.

FIG. 2 illustrates a side view of the example polygon ring laser amplifier of FIG. 1 between top and bottom plates.

FIG. 3 illustrates a top view of another example polygon ring laser amplifier suitable for use in association with disclosed master oscillator power amplifier laser systems.

FIG. 4 illustrates a cross-sectional view of an example amplifying reflection point suitable for use in association with disclosed polygon ring laser amplifiers and master oscillator power amplifier laser systems.

FIG. 5 illustrates example features of ellipsoidal mirrors.

FIG. 6 illustrates a curve reflecting relationships between mirror radii of curvature and optical cavity length.

FIG. 7 illustrates a top view of an example master oscillator power amplifier laser system.

DETAILED DESCRIPTION

The following detailed description illustrates embodiments of the disclosure and manners by which they can be implemented. Although the best mode of carrying out disclosed systems, apparatus and methods has been described, those of ordinary skill in the art would recognize that other embodiments for carrying out or practicing disclosed systems, apparatus and methods are also possible.

It should be noted that the terms “first”, “second”, and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Further, the terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.

Existing distributed gain (DG) laser resonators use many individual laser media disks/slabs. Laser power scalability by increasing the number of distributed laser media rods, disks, etc. can deteriorate beam quality. Further, each must be fabricated individually which adds to high fabrication and assembly cost.

Known laser disk media is fabricated either by crystalline growth or hot pressing of polycrystalline ceramic media. Disk size and material type is limited using the crystal growth method. Lutetium oxide (Lu₂O₃) is a desirable material for laser disk media but its extremely high melting point (2490° C.) has made crystalline growth prohibitive.

Depending on how they are used (such as thin disk laser applications), the disks must be individually cut, polished, coated, etc. which make them labor intensive. Subsequently they must be mounted and held in placed in the laser resonator.

Resonator round trip losses can be compensated for by making the laser disk thicker, and/or using multiple disks. However, thicker disks invite more individual disk heating gain inefficiencies and power scaling is limited by the ability to dissipate the heat. Designs with multiple individual disks invites fabrication and cooling complications.

Fiber laser power scale up is limited by length limitations of fiber laser modules. This makes multiple fiber laser modules necessary. Once again, this complicates thermal management. Present technology for power up scaling involves larger laser media crystal growth size which is inherently limited and increases in fiber optic module count which is equally problematic. Improvements in single mode fiber laser power performance may have reached a plateau.

Embodiments of the disclosure substantially eliminate, or at least partially address, problems in the prior art, enabling facilitated cooling, reduced volume and weight normally encountered with linear or axial laser resonator configurations, reduced diffraction losses, stable laser operation and correction for astigmatism and other optical aberrations at reflection points or elsewhere.

Embodiments of the disclosure employ synergistic operation of the monolithic laser resonator ring, polygon resonant operating mode, compact size, all ceramic laser gain, mirror, and substrate materials.

Embodiments of the disclosure provide a Distributed Gain—Polygon Ring Laser Amplifier (PRLA) that is a scalable, distributed gain, high energy laser (HEL) source that can compete with other exitsting distributed gain and fiber optic laser systems.

Disclosed PRLA's resonate at one of many convex-star polygon modes which, depending on design requirements, define the number of resonant reflection points and angles of incidence. With operation in a single resonator plane, the polygon resonant modes are solely defined by the pump laser input angle b with respect to the diameter of the substrate ring.

b=π/2(1−2×q/p)

Where q is the number of reflections and p is the density or the number of line segments a radius of the circumscribing circle would intersect without intersecting a vertex.

The ring substrate may include an inner surface with optical components at each polygon reflection point. Each reflection point may include a confocal or other resonator configuration. Cooling of the substrate may be approached externally with water or cryogenics and/or internally using refractive index matching fluids.

Additional aspects, advantages, features and objects of the disclosure will be made apparent from the drawings and the detailed description of the illustrative embodiments construed in conjunction with the appended claims that follow. It will be appreciated that described features are susceptible to being combined in various combinations without departing from the scope of the disclosure as defined by the appended claims.

FIGS. 1 & 2 illustrate an example polygon ring laser amplifier 100 suitable for use in association with disclosed laser systems. Substrate ring 110 of amplifier 100 provides a foundation or support for interior reflection points and contributes to isolation of laser beams from the environment. A laser entry point 111 is provided to the exterior of substrate ring 110 to allow for pumping laser light into the interior at an angle b defined by number of reflection points. A laser exit point 113 is provided to substrate ring 110 in the region of a reflection point configured to return light to the region of laser entry point 111. Laser entry point 111 and laser exit point 113 may be configured to respectively receive and output light beams at any of a variety of angles. Example angles include but are not limited to 90° to the ring resonator plane such as from above or below.

The angular position of each reflection point or segment is determined by the polygon resonant mode. Referring to FIG. 1 , for a 5/2 polygon mode, 5 refers to the number of points of the star polygon whereas 2 refers to the density which is the number of line segments a radius of the circumscribing circle would intersect without intersecting a vertex. In other words, it is the representation of the relative size of the circumscribing circle of the five outer vertices and the circumscribing circle of the five inner vertices.

FIG. 1 also illustrates an example beam path for a five-mirror resonator by which a beam would reflect from a mirror at M1 to a mirror at M4 to a mirror at M2 to exit point 113 which is at or near a mirror at M5 to a mirror at M3 and/or other region at or near laser entry point 111. Each mirror position may be diamond machined to a finish suitable for chemical vapor deposition (CVD) coating of a laser mirror at 99.99% reflectivity over the range of wavelengths between 0.8 and 3.0 microns. In an example, the mirrors are equally spaced around the inner surface of substrate ring 110. In the case of five mirrors, one mirror would be placed every 72°.

Holes 115 allow for insertion of fasteners 250 to secure top and bottom plates 210 and 230 to substrate ring 110 to seal polygon ring laser amplifier 100. The plates 210 and 230 may seal polygon ring laser amplifier 100 in accordance with a ConFlat configuration. The seal mechanism includes a knife-edge 117 that is machined below the flange's flat surface. As the bolts 250 of a flange-pair are tightened, knife-edges 117 make annular grooves on each side of a soft metal or fluorocarbon (Viton) gasket.

Substrate ring 110 includes one or more materials that are electrically insulative, have high thermal conductivity, have a low coefficient of thermal expansion and have a high melting point.

In an example, the electrical resistance of substrate ring 110 is 10¹⁰ ohm-meters.

The single monolithic substrate ring, made of a material with high thermal conductivity and provided in a vacuum sealable configuration, makes thermal management more efficient. Liquid cooling of provided laser media elements and/or optical components from the front and backside, can be more simply accomplished. The thermal conductivity supports high heat transfer away from active mirror and/or polygon laser disk segments thus significantly reducing thermal lensing and birefringence effects of laser media to reduce loss of round-trip gain. In an example, the thermal conductivity is greater than 300 W/mK.

The low thermal expansion coefficient also reduces thermal lensing of active mirror segments provided to substrate ring 110. In an example, the thermal expansion coefficient is less than 16×10⁻⁶/K. In a further example, the thermal expansion coefficient is less than 8×10⁻⁶/K.

With a high melting point, the material of substrate ring 110 will not deteriorate during any sintering of a laser media after being vapor deposited. In an example, the melting point is greater than 2000° C.

In an example, the one or more materials of substrate ring 110 include ceramic. In a further example, the ceramic is aluminum nitride (AlN) which has an electrical resistivity of 10¹² ohm-meters, a thermal expansion coefficient of about 4.5×10⁻⁶/K and a thermal conductivity of 321 W/(m K). In another example, the substrate ring may be formed from diamond which has a thermal conductivity of greater than 1800 W/(m K) and a thermal expansion coefficient of 1.1-2.6×10⁻⁶/K

Subjecting the inner surface of substrate ring 110 to a crystallization process by sintering or annealing of a laser mirror thereon reduces pores and grain size of the substrate. An example substrate ring exhibits a roughness of less than 1.0 nm rms at the inner surface.

Power up scaling of polygon ring laser amplifiers may be accomplished by, while maintaining spatial periodicity, increasing the number of reflection points to thereby increase the number of laser media nodes. FIG. 3 illustrates a top view of another example polygon ring laser amplifier 300 suitable for use in association with disclosed polygon ring laser amplifiers. Polygon ring laser amplifier 300 is scaled up from polygon ring laser amplifier 100 and supports employing a 9/4 star polygon beam path having nine reflection points. In an example, the mirrors M6-M14 of the 9/4 star polygon are equally spaced around the inner surface of substrate ring 310. In the case of nine mirrors, one mirror would be placed approximately every 40°.

Polygon ring laser amplifier 300 similarly includes a laser entry point 311 provided to substrate ring 310 to allow for pumping laser light into the interior at an angle b. A laser exit point 313 is provided to substrate ring 310 in the region of the reflection point configured to return light to the region of laser entry point 311. Holes 315 allow for insertion of fasteners 250 to secure top and bottom plates 210 and 230 to substrate ring 310 to seal polygon ring laser amplifier 300. Laser entry point 311 and laser exit point 313 may be configured to respectively receive and output light beams at any of a variety of angles. Example angles include but are not limited to 90° to the ring resonator plane such as from above or below.

Additional power up scaling is accomplished by, for example, doubling the pump power to twice the area on the active mirror while keeping the laser media thickness and doping level constant. The PRLA resonator would be modified to accommodate the increased mode size on the active mirror area. The mirror size would naturally be limited by a particular number of reflection points for a given substrate inner diameter. Increasing the size of a PRLA resonator is only limited by the availability of the substrate material and CVD capacity both of which can be scaled up accordingly.

Coupled with an input laser pump 710 configured to pump, direct or fire light to the interior of the substrate ring, the configuration becomes a Distributed Gain Master Oscillator Power Amplifier (DG-PRLA-MOPA).

FIG. 4 illustrates a cross-sectional view of an example reflection point 400 suitable for use in association with disclosed polygon ring laser amplifiers and master oscillator power amplifier laser systems. Reflection point 400 is configured to reflect light incident thereon from an input pump laser or one or more other reflection points to an output coupler or one or more additional reflection points. Each reflection point includes a laser mirror 450 for coupling to a substrate ring (such as substrate rings 110 or 310), laser media 430 provided to laser mirror 450 and an anti-reflective 410 coating provided to laser media 430.

Laser mirror 450 includes layers of materials with high laser damage thresholds. In an example, laser mirror 454 includes a layer of diamond-like carbon (Cd) 458 and a layer of pure Lu₂O₃ 454 which may be provided to a substrate ring by chemical vapor deposition. Diamond-like carbon exhibits an example refractive index of 2.418. Pure Lu₂O₃ exhibits a refractive index between 1.8 and 1.94. In an example, the diamond-like carbon is an amorphous carbon. In a further example, the diamond-like carbon is a hydrogenated amorphous carbon. The Lu₂O₃ may be sintered or annealed after being provided to the substrate ring to subject the substrate to a crystallization process to improve clarity by reducing pores and grain size. In another example, laser mirror 450 is comprised of layers of TiO₂ and SiO₂.

Because of the angular approach of the laser beam with a polygon ring laser master oscillator power amplifier, correction for astigmatism is designed into the mirror form. Each laser mirror 450 may be provided with an exterior surface for coupling with a substrate ring and an interior ellipsoidal surface configured to be directed with foci towards the interior of the ring. FIG. 5 illustrates example features of ellipsoidal mirrors suitable for use in association with disclosed polygon ring laser amplifiers. An input beam is reflected from each mirror M to a reflected beam. Ellipsoidal mirrors have two radii of curvature. A first mirror radius, R_(MT), is defined within a tangential plane defined by normal N_(T). A second mirror radius, R_(MS), is defined within a sagittal plane defined by normal N_(S). This ellipsoidal configuration supplies correction for astigmatism and diffraction losses in the beam toward single mode operation.

The sizes of the radii of curvature affect the beam output from the mirror. FIG. 6 illustrates a curve reflecting relationships between mirror radii of curvature and cavity length wherein:

$g_{1} = {{1 - {\frac{S\prime}{R_{T}}{and}g_{2}}} = {1 - \frac{S\prime}{R_{S}}}}$

While other configurations such as plane-parallel A, concave-convex C, concentric D or hemispherical F may be suitable, in an example, pairs of mirrors 450 represent a confocal configuration B where g₁=g₂=0. Here the radius of curvature R_(MT) is equal to radius R_(MS) (FIG. 5 ) which is equal to the optical path length S′ or the distance between pairs of mirrors.

Returning to FIG. 4 , laser media 430, which may include one or more ceramics, may be provided to the laser mirror 450 by chemical vapor deposition. In an example, laser media 430 includes Lu₂O₃. The Lu₂O₃ may be doped with rare earth minerals and/or heavy rare earth elements either independently or as mixtures. In a further example, the Lu₂O₃ is doped with ytterbium Yb in the form of ytterbim oxide. In another example, the reflection points are coated via laser-enhanced chemical vapor deposition with rare earth doped, polycrystalline, sesquioxide laser media. While laser media 430 may be provided in any of a variety of dimensions suitable for providing a gain to an incoming beam, in an example, laser media 430 is provided in a thickness of between about 100 and about 500 microns.

In an example, anti-reflective coating 410 exhibits less than 5% reflectivity over the range of wavelengths between 800 nm and 3.0 microns. In a further example, anti-reflective coating 410 exhibits 0.5% reflectivity or less. Anti-reflective coating 410 may include diamond-like carbon which may be applied by conventional chemical vapor deposition.

Unlike a typical thin laser disk system, the present individual laser media and/or mirror forms may be precision machined into a single substrate ring thus eliminating the fabrication, mounting, and alignment of individual laser disks.

Application of an undoped laser media end cap to the face of an active mirror may suppress amplified spontaneous emissions (ASE) and parasitic lasing which can significantly reduce laser power outputs. At the same time, the undoped cap may provide additional mechanical stabilization to avoid stress fractures and reduce thermal lensing. With the presence of an undoped cap, power scalability can be supported up to very high power levels.

Application of an undoped cap to a typical planar faced thin laser disk or an elliptically shaped active mirror by bonding processes creates a weak point at the interface and increases operational problems. The PRLA utilization of CVD coated active mirrors provides for flexibility in the amount of dopant present to yield an even more functional composite form of laser media.

FIG. 7 illustrates a top view of an example master oscillator power amplifier laser system 700. Laser system 700 includes a polygon ring laser amplifier 100 having inner and outer surfaces, top and bottom cover plates/flanges configured for vacuum sealing with a substrate ring, and a plurality of reflection points spaced around the inner surface of the substrate ring configured to reflect light from an input pump laser 710 to an output coupler 730.

In an example, the reflection points include segmented ellipsoidal mirrors. When laser pumped, each segment will behave like a pseudo-thin laser disk, and via distributed gain, has the potential of very large amplification capabilities. In an example, gains over a factor of 10 may be realized with a single active mirror with output power at 10 kW or greater. In another example, gains greater than 100 may be realized with cryogenically cooled and/or undoped cap active mirrors.

An input beamsplitter may be provided to facilitate direction of an input beam to a first reflection point to begin a trip through polygon ring laser amplifier 100. In an example, the input beamsplitter has a coating configured to cause near 100% reflectivity of a 975 nm pump beam while causing a 1080 nm laser emission beam to pass through unreflected. In this example, except for a small amount of leakage, all of the input 975 nm pump beam is reflected into the ring resonator plane.

The beam reflected by the input beamsplitter will make a round trip reflection from all active reflection points during which the beam will experience an amplitude gain. Upon each reflection, the wavelength of the incident beam is naturally down-converted to a longer wavelength in accordance with solid state laser-media-stimulated emission. There will be some unconverted light especially during some of the first active mirror reflections. After one round trip, most of the input beam has been converted to an amplified, longer wavelength light with very little left over which would occur for each incoming packet of light. The input beamsplitter may be configured to direct light from an input pump laser pumping light into the amplifier at any of a variety of angles. Example angles include but are not limited to 90° to the ring resonator plane such as from a top or bottom surface of amplifier 100.

An output beamsplitter may be provided to facilitate direction of a reflected beam out from polygon ring laser amplifier 100. In an example, the output beamsplitter reflects 5-10% of the amplified longer-wavelength light as an output while the remaining 90-95% of the longer-wavelength light will pass through the output beamsplitter to the input beamsplitter which then input beamsplitter will pass through again to continue circulating internally. The output beamsplitter may be configured to direct light from a reflection point out from the amplifier at any of a variety of angles including but not limited to 90° to the ring resonator plane.

The vacuum sealable construction of laser system 700 facilitates fluid cooling of the entire inner and outer surfaces of the substrate ring. A space 750 for cooling fluid may be provided between the top and bottom cover plates, the outer surface of the substrate ring and a system exterior shell 760. A cooling fluid may be provided to the space in thermal communication with the outer surface of the substrate ring. Alternatively or additionally, laser system 700 may provide for interior cooling by one or more refractive-index-matching fluids contained within the substrate ring where they would be traversed by beams reflected between reflection points.

Further, the vacuum seal supports vapor deposition of the primary and other supportive optical elements, e.g. saturable absorber for ultra-short laser performance. Vapor deposition of saturable absorber mirror (e.g. graphene) devices may be employed to mode lock. Optical components can be introduced into the beams as well, especially at the inner circle area.

For laser-enhanced chemical vapor deposition and other uses, vacuum feed-thru ports may be designed into the blank sealing flanges to provide pump down, liquid cooling, metering, and other access to the laser cavity interior once sealed.

Embodiments of the disclosure are susceptible to being used for various purposes, including, though not limited to, amplifying an input beam such as a laser while enabling facilitated cooling, reduced volume and weight, reduced diffraction losses, stable laser operation and correction for astigmatism and other optical aberrations. The polygon ring construction makes access to Whispering gallery mode (WGM) action at its center a possible optical avenue that can be exploited.

Modifications to embodiments of the disclosure described in the foregoing are possible without departing from the scope of the disclosure as defined by the accompanying claims. Expressions such as “including”, “comprising”, “incorporating”, “consisting of”, “have”, “is” used to describe and claim disclosed features are intended to be construed in a non-exclusive manner, namely allowing for items, components or elements not explicitly described also to be present. Reference to the singular is also to be construed to relate to the plural. 

What is claimed is:
 1. A distributed gain polygon ring laser amplifier, comprising: a substrate ring having inner and outer surfaces; and a plurality of reflection points spaced around the inner surface of the substrate ring and configured to reflect light from an input pump laser to an output coupler.
 2. The amplifier as set forth in claim 1, wherein the substrate ring further includes a material having thermal conductivity of greater than 300 W/mK.
 3. The amplifier as set forth in claim 1, wherein the substrate ring further includes a material having a melting point greater than 2000° C.
 4. The amplifier as set forth in claim 1, wherein each of the reflection points includes a laser mirror coupled to the substrate ring, laser media provided to the laser mirror and an anti-reflective coating provided to the laser media.
 5. The amplifier as set forth in claim 4, wherein the laser mirror is configured with an interior ellipsoidal surface directed with foci towards the interior of the substrate ring.
 6. A master oscillator power amplifier laser system, comprising: a substrate ring having inner and outer surfaces; top and bottom cover plates configured for vacuum sealing with the substrate ring; an input pump laser configured to direct light into the substrate ring; an output coupler; and a plurality of reflection points spaced around the inner surface of the substrate ring configured to reflect light from the input pump laser to the output coupler.
 7. The system as set forth in claim 6, wherein the substrate ring further includes a ceramic material.
 8. The system as set forth in claim 6, wherein the laser mirror is configured with an interior ellipsoidal surface directed with foci towards the interior of the substrate ring.
 9. The system as set forth in claim 6, further comprising a cooling fluid contained within a region defined between the top and bottom cover plates and the outer surface of the substrate ring.
 10. The system as set forth in claim 6, further comprising refractive-index-matching cooling fluids at the interior of the substrate ring.
 11. A ring laser reflection point, comprising: a laser mirror; an anti-reflective coating; and laser media sandwiched between the laser mirror and the anti-reflective coating.
 12. The laser ring reflection point as set forth in claim 11, wherein the laser mirror includes layers of diamond-like carbon and Lu₂O₃.
 13. The laser ring reflection point as set forth in claim 11, wherein the laser mirror includes layers of TiO₂ and SiO₂.
 14. The laser ring reflection point as set forth in claim 11, wherein the laser mirror includes an interior ellipsoidal incident surface and an exterior surface configured for coupling with a laser ring substrate.
 15. The laser ring reflection point as set forth in claim 11, wherein the laser media includes a ceramic.
 16. The laser ring reflection point as set forth in claim 11, wherein the laser media includes Lu₂O₃.
 17. The laser ring reflection point as set forth in claim 11, wherein the laser media includes Lu₂O₃ doped with rare earth minerals.
 18. The laser ring reflection point as set forth in claim 11, wherein the anti-reflective coating exhibits less than 5% reflectivity.
 19. The laser ring reflection point as set forth in claim 18, wherein the anti-reflective coating exhibits no more than 0.5% reflectivity.
 20. The laser ring reflection point as set forth in claim 11, wherein the anti-reflective coating includes diamond-like carbon. 